Studies are proposed which will investigate the following aspects of lipid metabolism in the alveolus: (1) synthesis of fatty acids and exchange of acyl groups between triglycerides and cholesterol esters and phospholipids with emphasis on the role these neutral lipids may play in the maintenance of surface-active disaturated phosphatidyl choline in the alveolar lining, (2) the role of the type II alveolar epithelial cell in synthesis, exchange and secretion of cholesterol and cholesterol esters and their modification in animals made hypolipoproteinemic by the drug AMPP, (3) the metabolic basis for intraealveolar accumulation of saturated phospholipids induced by the delta-7-sterol reductase inhibitor AY-9944. Experiments will employ lung slices, and suspended alveolar macrophages and type II alveolar cells, both freshly isolated and in short-term culture.